In an effort to meet the increasing demands of vehicle operators and occupants, vehicle manufacturers and their suppliers are increasingly designing and manufacturing vehicles with additional and improved vehicle stability management (VSM) features. One such VSM feature that has been incorporated into a number of vehicle types is anti-lock braking, which can take a variety of forms. Another such feature is traction control (i.e., TC).
Vehicle anti-lock brake systems are designed to maximize the ability of a vehicle operator to bring a vehicle to a controlled stop on any type of road surface. The system accomplishes this goal by preventing the vehicle brakes from prematurely halting vehicle wheel rotation, or "lecking" the vehicle wheels, regardless of the road surface and the pressure applied to the brake pedal by the vehicle operator.
Typical vehicle anti-lock brake systems (i.e. ABS) include vehicle wheel speed sensors for providing inputs to an anti-lock brake system control unit. The control unit controls anti-lock brake system control valves interposed between the brake master cylinder and the individual wheel brakes of a hydraulic brake circuit. Such control valves include isolation valves and dump valves. The control valves, in turn, regulate hydraulic brake fluid pressure in the individual wheel brakes to implement anti-lock braking. ABS pump(s) pumps fluid from accumulator(s) to the various control valves.
In operation, one or more of the vehicle wheel speed sensors provides input to the control unit for subsequent determination of not only wheel speed but also the vehicle speed. The control unit monitors the vehicle and vehicle wheel speeds for an indication of an anti-lock braking event. First, based upon the vehicle speed, the control unit typically determines a slip threshold. Using the vehicle velocity as a reference, slip threshold may be expressed as the difference between a selected velocity and the vehicle velocity.
In most ABS and/or TC control systems, wheel speed sensor continuity is self-tested by the ABS and/or TC microcontroller. Some systems measure the bias voltage which reflects the sensor resistance while others simply flip-flop the input circuit with a test pulse. However, these systems do not solve the problem where the sensor generates an incorrect number of analogic pulses which, in turn, are converted to a digital pulse train having a corresponding incorrect number of digital pulses. In this situation, the sensor may be bad, incorrectly installed, or may have come loose.